The medial and lateral menisci are biconcave, generally C-shaped wedges of fibrocartilage interposed between the condyles of the femur and the tibia. Together, the menisci act as crucial stabilizers, providing a mechanism for force distribution, and a lubricant between the tibia and the femur. Without functional menisci, stress concentration occurs in the knee in conjunction with abnormal joint mechanics. These phenomena can result in premature development of arthritis.
In the past, the treatment of choice for injured or diseased menisci was partial or complete excision or replacement of the meniscus. Unfortunately, meniscectomy is often followed by degenerative changes within the knee joint. Replacement of injured menisci in an otherwise healthy knee joint, however, may prevent arthritic changes and may stabilize the joint. In diseased joints, replacement of the meniscus may reduce the progression of the disease process. Allografting or meniscal transplantation have been performed in dogs and humans. However, these approaches have been only partially successful over the long term due to the host's immunologic response to the graft, failures in the cryopreservation process, and failures of the attachment sites.
Menisci have also been replaced by prostheses composed of permanent artificial material. Such prostheses have been constructed of purely artificial materials in order to minimize the possibility of an immunological response thereto. The use of such artificial materials is believed to be advantageous because it permits construction of a structure which can withstand the high and repeated loads which are encountered in the knee joint, and because it can alter the joint mechanics in beneficial ways that biological materials supposedly would not tolerate.
For example, a Teflon.RTM. net has been used to replace the resected meniscus of a dog upon which fibrous ingrowth or regeneration was observed, although accompanied by significant chondral abrasion. Resilient materials such as silicone rubber or Teflon.RTM. with reinforcing stainless steel or nylon strands have also been employed in the construction of prosthetic menisci (U.S. Pat. No. 4,502,161). Meniscal components have also been generated from resilient plastic materials (U.S. Pat. No. 4,085,466). In addition, reconstruction of menisci following lesioning has been attempted withcarbon-fiber-polyurethane-poly (L-lactide) with some success (Leeslag et al. (1986) Biological and Biomechanical Peformance of Biomaterials (Christel et al., eds.) Elsevier Science Publishers B.V., Amsterdam, pp. 341-352).
The replacement of meniscal tissue with structures consisting of permanent artificial materials, however, has generally been unsuccessful. This lack of success is due principally to the fact that opposing articular cartilage of human and animal knee joints is fragile. The articular cartilage in the knee joint will not withstand abrasive interfaces, nor compliance variances from normal, which eventually result from implanted artificial prosthetic menisci. In addition, joint forces are multiples of body weight which, in the case of the knee and hip, are typically encountered over a million cycles per year. Thus far, permanent artificial menisci have not been composed of materials having natural meniscal properties, nor have they been able to be positioned securely enough to withstand such routine forces.
Stone (U.S. Pat. Nos. 5,007,934, 5,116,374, and 5,158,574) describes a prosthetic, resorbable meniscus comprising biocompatible and bioresorbable fibers, such as natural polymers, and methods for fabricating such prosthetic menisci. In addition, Stone describes methods of regenerating meniscal tissue by implanting the resorbable prosthetic meniscus into a human knee.